There continues to be a need for improved barrier coatings in that considerable energy loss occurs through air leakage in the wall assembly of heated or air conditioned buildings. In order to minimize excess air and water intrusion, building codes often require the use of a barrier on interior walls such as 4-mil poly and/or a building rape sheeting such as Tyvek™ from Dupont (Willington, Del.) on exterior walls. Ideally, to reduce the incidence and prevalence of conditioned internal-air from exiting into the wall cavity and having moisture condense on the inside surface of the exterior wall; the building rape should have effective moisture vapor permeability. It is generally understood that decreased moisture vapor permeability is often undesirable in that high humidity conditions within the wall assembly increases the potential of mold development. A drawback with building rapes like Tyvek™ is that while these sheeting materials are very hydrophobic, providing a water-resistant barrier, they are ineffective as air barriers. Tyvek™ was measured at 0.013 cfm/sf at 0.3 inch water pressure, with the acceptable limit for air barriers being less than 0.004 cfm/sf at 0.3 inch water pressure. In order for sheeting type barriers to be effective, they must be continuous without penetrations or openings. In practice, there are often improperly sealed edge joints, taped joints that fail due to cold weather application, and wind blown open joints allowing for water and air penetration. Any leakage point from an improperly sealed sheeting edge joint will allow air and water to flow behind the sheeting all across the building face with multiple opportunities for penetration through the wall assembly. The application cost is high due to the need for cutting, nailing or stapling, and taping.
A preferred barrier film/coating should have an effective combination of properties including low air permeability, high moisture vapor permeability, and good water resistance. For example, a barrier film/coating having moisture vapor permeability greater than 1 U.S. Perm is considered a breathable film. Furthermore, Canadian Standard CAN 2-51.32-M77 requires that a breathable barrier have moisture vapor permeability greater than 3 U.S. Perms. In continuation, the acceptance criteria for a breathable barrier film/coating as specified by the I.C.C. Evaluation Services, Inc. is a moisture vapor transmission rate greater or equal to 35 grams/m 2.24-hours (equivalent to 1.44 U.S. Perms tested at 100° F. and 75 percent relative-humidity differential).
It is known that liquid compositions are useful for the preparation of barrier coatings. Such compositions are easily applied onto wall surfaces, at the construction site, and often have processing and property advantages compared to building rapes. For example, several references are described below:
U.S. Pat. No. 4,716,188 (Mariusson, et al.) discloses a water-repellent and vapor-permeable solvent-based paint. A drawback with the barrier-composition is that volatile organic solvents are required. Such solvents are often less environmentally friendly compared to water-based barrier compositions.
U.S. Patent Application 20060155031 (Wiercinski; Robert A.) discloses a liquid applied air barrier which uses at least one water-soluble polymer for providing a vapor permeable membrane. A drawback of this invention is that the water-soluble polymer would be water leachable under wet conditions, and causes significant water absorption of up to 50%. The dried membrane also is applied at 20-100 mils thickness, which would be very difficult in a plant application to building panels, and very costly compared to the sheeting materials available.
Sto Corporation (Atlanta, Ga.) markets a water-based barrier composition (e.g., Sto Gold Coat) that can be spray applied and then dried to provide a barrier coating having a reported moisture vapor transmission rate of 5.6 U.S. Perms (wet cup, method B). The drawback with the water-based composition is that crystalline silica is used in the formulation, as reported in the material safety data sheet, which is a suspected carcinogen when inhaled. The moisture vapor permeability also drops significantly by the dry cup method at 37.8 centigrade. It was tested at 0.33 metric perms (0.50 U.S perms) at 200 microns wet.
Therefore, there remains a need for water-based compositions, having effective moisture vapor permeability when used as an air and water barrier coating, which are substantially free of volatile organic solvents and suspected carcinogens.